It is known in the art that molecular sieve materials may be deposited on porous supports to form thin-layered membranes that have characteristics making them useful in the separation of gaseous mixtures. A number of publications report various methods of manufacturing such thin-layered membranes. For instance, U.S. Pat. No. 5,723,397 discloses a method of depositing from a synthesis solution a molecular sieve layer upon a support. The synthesis mixture of this method comprises a silica source and an organic structure directing agent and may be suitable for forming molecular sieves such as aluminosilicate, aluminophosphate, silicoaluminophosphate, metalloaluminophosphate or metalloaluminohosphosilicate. The support is immersed into the synthesis mixture, and a layer of crystallized molecular sieve particles is formed on the support. During this synthesis step, the crystallization temperature is stepwise or continuously increased over a period that may, for example, range from 24 hours to 7 days, and, preferably, this period is in the range of between 3 and 6 days. The membrane may be used to carry out separations of hydrocarbons and alcohol from aqueous streams. There is no indication that the membranes may suitably be used in the selective separation of carbon dioxide from methane or other hydrocarbons. The method also requires the temperature of the synthesis mixture to be increased during the crystallization in a stepwise fashion over the above-noted significantly long time period. There is no teaching of the separation characteristics of the membrane except with respect to the amount of defects found in the membrane layer.
U.S. Pat. No. 6,177,373 discloses a method of preparing a molecular sieve monolayer upon a substrate. In this method, a thin, molecular sieve film is deposited on the surface of a porous substrate by contacting it with a molecular sieve synthesis mixture under hydrotreatment conditions. The heating time may be in the range of from 1 hour to 6 days. The membrane product of the '373 patent may be used in hydrocarbon separations, but there is no mention of the application of the membrane product in the selective separation of carbon dioxide from lower molecular weight hydrocarbon compounds or of it having the capability to selectively separate carbon dioxide from lower molecular weight hydrocarbons, such as methane, ethane and the like. Moreover, there is no suggestion that a supported silicoaluminophosphate (SAPO) membrane may be made from a SAPO synthesis mixture, using a shortened crystallization time, which exhibits high performance carbon dioxide/lower molecular weight hydrocarbon performance characteristics. The required crystallization times for preparing the molecular sieve membranes taught by the '373 patent are significantly long, and, while the '373 patent may suggest that the membranes may be continuous and dense, there is no suggestion that they can have high performance in the selective separation of carbon dioxide from lower molecular weight hydrocarbons.
U.S. Pat. No. 7,316,727 discloses a method of preparing a SAPO membrane by placing a porous support within a crystallization synthesis gel under crystallization synthesis conditions to deposit on the surface thereof one or more layers of SAPO crystals to form a continuous, gas-tight membrane. This patent teaches that long crystallization times in the range of from about 15 to about 25 hours are required for each synthesis step in order to form the continuous, gas-tight membranes, and it further suggests that the formation of several layers of crystals by the application of multiple synthesis steps may be necessary in order to form a continuous layer that is impermeable (before calcination thereof) to nitrogen. There is no indication in the '727 patent that a single, rapid crystallization step may be used to form a thin, gas-tight membrane on the support, nor is there a suggestion that a single-step, rapid crystallization will provide for a membrane that exhibits characteristically high performance in the separation of carbon dioxide from methane relative to membranes made by using longer crystallization times.
There is a need for improved methods to economically manufacture supported molecular sieve membranes that have high-performance gas separation properties.